An Ion Mobility Spectrometer (IMS) is a device primarily used for detecting atoms and molecules in a given sample of gas. The theory behind ion mobility spectrometry is that every ionized atom or molecule has a unique size, shape and mass-to-charge ratio, so that when an electric or magnetic force is applied to the ionized atom or molecule, constrained by collisions with the host gas, it will travel at a certain velocity. This velocity can be measured, and thereby the type of atom or molecule can be identified.
The IMS of the prior art is essentially a cylinder operating at atmospheric pressure. Sample gas enters the cylinder at one end, is charged, then is moved through the cylinder by an electric field, and measured at the opposite end. The portion of the cylinder where the gas enters is called the ion molecule reaction region. This section, known as the drift region, is separated from the rest of the cylinder by a control grid. The control grid is a series of parallel wires with alternating charge. The grid thereby keeps most charged particles effectively contained in the ion molecule reaction region until they are lost by contacting a surface.
A series of metal rings along the cylinder, referred to as guard rings, provide a series of electric fields, which create an electrical gradient through the center of the cylinder. This field is what propels the ions though the drift gas within the IMS cylinder when the control grid is opened. The length of time it then takes ion to reach the collector electrode at the opposite end can be precisely measured in terms of milliseconds. Since each ion has a unique size, shape and mass-to-charge ratio, the length of time through the IMS is unique to each particle. A specific compound can be determined in terms of parts per million.
The detection of gasses in the parts per billion, however, is a sensitive process. The less concentrated a particle is, the harder it its to detect over the background signals, referred to as noise. Also, if a particular ionized atom or ionized molecule has a flight time through the IMS that is similar to a more abundant gas, its signal can be lost if the resolution of the system is not accurate enough.
The way to correct this problem is to repeat the measuring process tens, hundreds or even thousands of times, and is called signal averaging. By doing this a signal can become readily apparent over background noise, even at very low concentrations. However, if the system is not accurate enough, a weak signal can still get lost next to a strong one. Further, it is not always practical to repeat the detection process hundreds or thousands of times, such as when testing for toxic gasses in real time.
Therefore anything that can help to improve signal to noise ratio and signal resolution would be useful and needed.
One cause of signal deterioration is the guard rings themselves. The guard rings form an inner space in which the ions pass. The middle of the inner space is referred to as the linear region. This region has a diameter approximately half that of the inner space the guard rings form. In this space the ions travel in a linear path. As the flight of an ion starts closer to the guard rings, halfway between the guard rings and the centerline of the cylinder, the ions start to drift more towards the edges. This less linear electric field region is caused by the proximity of the ions to the guard rings. The closer the ion starts to the guard rings, the greater the sidewise drift and the longer the path length. This will cause signals to be less sharp as some of the measured ions and molecules are taking longer to reach the collector electrode as they travel at an angle rather than a straight line. Further, some of the ions drift to such an extent that they hit the grid mounting device or other obstruction and are totally lost for signal measuring purposes.
Attempts have been made to correct this problem. One such solution is to block, or otherwise not read, the ions that are not traveling in the linear zone. This ensures a more uniform flight time of the measured ions, and creates a sharper peak. However, a large number of the ions are blocked from being read by the collector, and this lowers the signal to noise ratio, since the area of the outer less linear drift region is substantially larger than the surface area of the inner linear drift region.
What is needed is a way of improving the electric field linearity so that like ions passing through all portions of the drift region exhibit the same time of flight and thereby the signal resolution can be improved without sacrificing the signal to noise ratio.